A magnetic random access memory (MRAM) is expected and actively developed as a nonvolatile memory capable of performing a high-speed operation and rewriting an infinite number of times. In the MRAM, a magneto-resistance effect element is integrated in a memory cell and data is stored as orientation of magnetization of a ferromagnetic layer in the magneto-resistance effect element. Some types of MRAMs are proposed to meet methods of switching the magnetization of the ferromagnetic layer.
A current-induced magnetic field write type MRAM is one of the most general MRAMs. In this MRAM, a wiring line that a write current passes through is installed on the periphery of the magneto-resistance effect element and the magnetization direction of the ferromagnetic layer in the magneto-resistance effect element is switched by a current magnetic field that occurs due to the passage of the write current. This MRAM can theoretically perform writing at a speed of 1 nanosecond or less and thus, is suitable for a high-speed MRAM. For example, a success of an operation at 250 MHz is demonstrated in one report (N. Sakimura et al., “A 250-MHz 1-Mbit Embedded MRAM Macro Using 2T1MTJ Cell with Bitline Separation and Half-Pitch Shift Architecture”, Solid-State Circuits Conference, 2007, ASSCC' 07, IEEE Asian p. 216). Further, a circuit configuration suitable for an operation at 500 MHz is proposed (N. Sakimura et al., “MRAM Cell Technology for Over 500-MHz SoC”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol, 42, 2007, p. 830).
However, a magnetic field for switching magnetization of a magnetic body having secured thermal stability and resistance to external disturbing magnetic field is generally a few dozens of [Oe]. In order to generate such magnetic field, a large write current of about a few mA is needed. Even the smallest write current among reported write currents is about 1 mA (H. Honjo et al., “Performance of write-line-inserted MTJ for low-write-current MRAM cell”, 52nd Magnetism and Magnetic Materials Conference 2007 (MMM 2007), p. 481). When the write current is large, the chip area is necessarily large and power consumed for writing increases. In addition, when a size of the memory cell is miniaturized, the write current further increases and scaling is not applicable.
As for other MRAMs, a spin-polarized current write type MRAM is exemplified. In this MRAM, a spin-polarized current is injected into a ferromagnetic conductor of the magneto-resistance effect element and magnetization is switched due to a direct interaction between spin of conduction electrons that bear the current and a magnetic moment of the conductor (hereinafter referred to as “spin transfer magnetization switching”). Generating of the spin transfer magnetization switching depends on a current density (rather than a current absolute value). Accordingly, when the spin transfer magnetization switching is utilized for data writing, as the size of the memory cell decreases, the write current is also reduced. In other words, the spin transfer magnetization switching method is excellent in scaling performance. When the write current is small, the chip area becomes small, enabling higher integration and larger structure. However, as compared to the current-induced magnetic field write type MRAM, a write time period tends to be longer (ex. 1 nanosecond or more).